The present invention relates to a polyethylene-2,6-naphthalate film for capacitors and, more particularly, to a polyethylene-2,6-naphthalate film which is excellent in heat resistance and mechanical strength and is useful for the dielectric of a chip type plastic film capacitor which requires a very thin film.
As is well known, as a film for plastic film capacitors, a biaxially oriented polyethylene terephthalate film or a biaxially oriented polypropylene film is conventionally used. Recently, a thinner film has been demanded in order to improve the dielectric properties of a capacitor. For example, an attempt has been made on a biaxially oriented polyethylene terephthalate film of 1.5 .mu.m thick. A conventional very thin film of polyethylene terephthalate or polypropylene, however, has disadvantage that it often produces breakage during vacuum deposition because of its low strength.
With the miniaturization of electric and electronic equipments, high-density electronic parts have been developed. As a useful means for densifying electronic parts, they have been produced in the form of chips. Among various kind of capacitors, only ceramic capacitors have hitherto been capable of being produced in the form of chips, but recently it is possible to produce a tantalum electrolytic capacitor and an aluminum electrolytic capacitor in the form of chips.
In the case of a plastic film capacitor, however, the heat resistance of a plastic film itself, which used as a dielectric, is low and when it is immersed in a bath of molten solder, critical defects such as the deformation of the base film, the degradation of the dielectric properties and the short-circuit between the lead wires are produced, thereby making it difficult to be produced in the forms of a chip. On the other hand, since a heat-resistance film now on the market is too large in thickness to obtain adequate capacity and too large in a dielectric loss tangent, and costs too much, it is unsuitable as the dielectric of a capacitor.
Accordingly, it is an object of the present invention to eliminate the above-described problems in the prior art and to provide a film which is excellent in strength, the soldering temperature resistance and the dielectric properties as a capacitor, and is not degraded much due to heat even if it is as thin as less than 2.5 .mu.m. As a result of the researches of the present inventor to achieve this aim, it has been found that a film having a degree of crystallinity of not smaller than 40% obtained by the biaxial orientation of polyethylene-2,6-naphthalate having an intrinsic viscosity of not smaller than 0.40 exhibits a Young's modulus of not smaller than 600 kg/mm.sup.2 both in the longitudinal direction (the machine direction) and in the transverse direction, and is excellent in the soldering temperature resistance and the dielectric properties as a capacitor even if it is as thin as 2.5 .mu.m or less, and that it is therefore suitable as a plastic film capacitor, in particular, as the dielectric of a chip type plastic film capacitor. The present invention has been accomplished on the basis of this finding.